Periodic (daily) Time Machine failures

I am running TM on an iMac with 10.5.8 to a Duo running 4.1.6. However I am getting a failure about once a day where TM pops up a dialog saying the network volume could not be mounted because of a problem with the username or password. It is very odd because it will do its hourly backup just fine for some period of time (a day or maybe a bit longer) before giving me this error. If I go to the TM Preferences I have to re-enter the username and password, then all is fine again for a day or so. This happened again just now and I snagged the Duo logs and also took a look at the Mac log. On there I see this series of possibly related messages:

Aug 16 17:07:40 Kobe kernel[0]: AFP_VFS afpfs_mount: /Volumes/ReadyNAS, pid 4993
Aug 16 17:07:40 Kobe Bonjour Mounter[267]: DiskAppearedCallback diskDescription = {\n DAVolumeKind = afpfs;\n DAVolumeMountable = 1;\n DAVolumeName = ReadyNAS;\n DAVolumeNetwork = 1;\n DAVolumePath = file://localhost/Volumes/ReadyNAS/;\n}
Aug 16 17:07:42 Kobe [0x0-0x14c14c].com.apple.systempreferences[4987]: chown: /Volumes/ReadyNAS/.001ec20ffc9e: Operation not permitted
Aug 16 17:07:43: --- last message repeated 1 time ---
Aug 16 17:07:43 Kobe kernel[0]: AFP_VFS afpfs_unmount: /Volumes/ReadyNAS, flags 0, pid 4998
Aug 16 17:07:43 Kobe Finder[161]: StatusMonitor::volumesChangedCallBack returned -47
Aug 16 17:07:43 Kobe kernel[0]: ASP_TCP do_thread_read: no reqInfo found for reqID 1
Aug 16 17:07:43 Kobe Bonjour Mounter[267]: DiskDisappearedCallback diskDescription = {\n DAVolumeKind = afpfs;\n DAVolumeMountable = 1;\n DAVolumeName = ReadyNAS;\n DAVolumeNetwork = 1;\n DAVolumePath = file://localhost/Volumes/ReadyNAS/;\n}
Aug 16 17:07:43 Kobe kernel[0]: ASP_TCP CancelOneRequest: cancelling slot 29 error 89 reqID 30 flags 0x9 afpCmd 0x14 so 0x7e7a330

In Duo afp.log
Aug 16 17:07:42 cnid_dbd[29190][logger.c:255]: I:Logger: doing log_setup, type 0, level 50, filename "/var/log/netatalk.log"
Aug 16 17:07:42 cnid_dbd[29190][logger.c:395]: D7:Logger: log_file_arr[0] now contains: {log_filename:/var/log/netatalk.log, log_file:(nil), log_level: 50}
Aug 16 17:07:42 cnid_dbd[29190][logger.c:398]: D5:Logger: log_setup[0] done
Aug 16 17:07:42 cnid_dbd[29190][main.c:210]: I:CNID: Setting uid/gid to 0/0
Aug 16 17:07:42 cnid_dbd[29190][main.c:309]: I:CNID: Startup, DB dir /c/.timemachine/.AppleDB

Any idea how I prevent this from happening? It means if I'm not in front of the Mac for awhile the TM backups stop.